Environmental Engineering Reference
In-Depth Information
form HCO
3
-
, and the carbonate buffering system would allow oceans to
retain a stable pH despite rising emissions. However, as the partial pressure
of carbon dioxide (pCO
2
) increases, the buffering capacity of seawater
decreases, in fact the ocean pH has been reduced by 0.1 since the start of
the industrial revolution, representing a 30% increase in the concentration
of H
+
ions (Caldeira and Wickett 2003, Key et al. 2004, Doney et al. 2009).
Effects of ocean acidifi cation on marine zooplankton
It is considered that the concentration of carbon dioxide in the surface
ocean was more or less in equilibrium with overlying atmosphere CO
2
;
however, researchers largely dismissed the potential impact on the ocean
biota because calcite, the CaCO
3
mineralogy of most marine calcifying
organisms, would remain supersaturated in the surface ocean (Doney et al.
2009). Since then, multiple studies consider ocean acidifi cation as a threat
to marine biota. Most of them found that:
• The calcifi cation rates of many shell-forming organisms respond to
the degree of supersaturation of CaCO
3
minerals (e.g.
,
Smith and
Buddemeier 1992, Kleypas et al. 1999).
• Aragonite, a more soluble CaCO
3
mineral in calcifying organisms,
may become undersaturated in the surface ocean within the early 21st
century (Feely and Chen 1982, Feely et al. 1988, Orr et al. 2005).
• The biological effects of decreasing ocean pH reach far beyond limiting
calcifi cation (e.g., Mayor et al. 2007, Fabry et al. 2008).
Therefore, in marine organisms, physiological processes such as growth,
development, metabolism, ionoregulation and acid-base balance, can be
affected by increases in CO
2
(Fabry et al. 2008, Pörtner and Farrell 2008,
Pörtner et al. 2004, Widdicombe and Spicer 2008) (Fig. 2). However, most
of the studies concerning ocean acidifi cation on marine organisms are
focused on calcifying organisms which produce CaCO
3
shells or skeletons
(Gattuso et al. 1998, Kleypas et al. 2006, Riebesell et al. 2000) and the reduced
availability of carbonate ions which affects calcifi cation processes. These
works include the dissolution of calcifying plankton but also the reduced
growth and shell thickness in gastropods and echinoderms and declining
growth of reef-building corals.
Effects on plankton food chain.
Ocean acidifi cation can produce indirect
impacts on zooplankton, since it may change the biochemical composition
of phytoplanktonic preys, affecting its nutritional food quality. Elevated
concentrations of CO
2
can encourage carbon fi xation in primary producers
and thus reduce the nutrient content relative to carbon (Rossoll et al. 2012).
Under elevated CO
2
scenario, enhanced carbon consumption relative to
nutrients, can produce changes in the phytoplankton stoichiometry thus
